Method of reducing a blocking artifact when coding moving picture

ABSTRACT

A method of coding a moving picture is provided that reduces blocking artifacts. The method can include defining a plurality of defining pixels S 0 , S 1 , and S 2 , which are centered around a block boundary. If a default mode is selected then frequency information of the surroundings of the block boundary is obtained. A magnitude of a discontinuous component in a frequency domain belonging to the block boundary is adjusted based on a magnitude of a corresponding discontinuous component selected from a pixel contained entirely within a block adjacent the block boundary. The frequency domain adjustment is then applied to a spatial domain. Or, a DC offset mode can be selected to reduce blocking artifacts in smooth regions where there is little motion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a moving picture process, and inparticular to a method for processing blocks of a moving picture toincrease a compression ratio and to improve coding efficiency.

2. Background of the Related Art

To efficiently compress a time variable video sequence, redundancy inthe temporal domain as well as in the two dimensional spatial domainmust be reduced. MPEG uses a discrete cosine transform (DCT) to reducethe redundancy in the two dimensional spatial domain and a motioncompensation method to reduce the redundancy in the temporal domain.

The DCT is a method of reducing the correlativity between data through atwo dimensional spatial transformation. Each block in a picture isspatially transformed using the DCT after the picture is divided intoblocks. Data that has been spatially transformed tends to be driven to acertain direction. Only a group of the data driven in the certaindirection is quantized and transmitted.

Pictures, which are consecutive in the temporal domain, form motions ofa human being or an object at the center of the frame. This property isused to reduce the redundancy of the temporal domain in the motioncompensation method. A volume of data to be transmitted can be minimizedby taking out a similar region from the preceding picture to fill acorresponding region, which has not been changed (or has very littlechange), in the present picture. The operation of finding the mostsimilar blocks between pictures is called a motion estimation. Thedisplacement representing a degree of motion is called a motion vector.MPEG uses a motion compensation-DCT method so that the two methodscombine.

When a compression technique is combined with a DCT algorithm, the DCTtransform is usually performed after input data is sampled in a unitsize of 8×8, and the transform coefficients are quantized with respectto a visual property using quantization values from a quantizationtable. Then, the data is compressed through a run length coding (RLC).The data processed with the DCT is converted from a spatial domain to afrequency domain and compressed through the quantization with respect tothe visual property of human beings, not to be visually recognized. Forexample, since eyes of human beings are insensitive to a high frequency,a high frequency coefficient is quantized in a large step size. Thus, aquantization table is made according to external parameters, such as adisplay characteristic, watching distance, and noise, to perform anappropriate quantization.

For the quantized data, the data having a relatively high frequency iscoded with a short code word. The quantized data having a low frequencyis coded with a long code word. Thus, the data is finally compressed.

In processing a moving picture as discussed above, blocks areindividually processed to maximize the compression ratio and codingefficiency. However, the individual process causes blocking artifactsthat disturb the eyes of human beings at boundaries between blocks.

Accordingly, various methods for reducing a blocking artifact in acoding system, which individually processes blocks, are presented. Forexample, attempts to reduce the blocking artifact by changing processesof coding and decoding have been implemented. However, this method ofchanging the processes of coding and decoding increases the amount ofbits to be transmitted.

Another method for reducing the blocking artifact is based on the theoryof projection onto convex sets (POCS). However, this method is appliedto only a still picture because of an iteration structure andconvergence time.

The blocking artifact is a serious problem in a low transmit rate movingpicture compression. Since a real-time operation is necessary in codingand decoding a moving picture, it is difficult to reduce the blockingartifact with a small operation capacity.

Consequently, the related art methods involve various problems anddisadvantages when reducing a blocking artifact created in coding amoving picture. A calculation for performing an algorithm iscomplicated, and the calculation amount and time become correspondinglylarge. Further, the blocking artifacts are not reduced in either complexregions or smooth regions in a picture. In addition, the amount of bitsto be transmitted increases.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for reducing ablocking artifact appearing when coding a moving picture thatsubstantially obviates one or more of the limitations and disadvantagesof the related art.

Another object of the present invention is to provide an MPEG-4 videocoding method that reduces a blocking artifact in a real-time movingpicture using a frequency property around boundaries between blocks.

A further object of the present invention is to provide a method forreducing a blocking artifact that increases a compression ration andincreases a coding efficiency.

To achieve these and other advantages in whole or in parts, and inaccordance with the purpose of the present invention as embodied andbroadly described, a blocking artifact reduction method includesdefining pixels centered around a block boundary and setting a defaultmode. Frequency information of the surroundings of the block boundary isobtained for each pixel using a 4-point kernel. A magnitude of adiscontinuous component that belongs to the block boundary is adjustedin a frequency domain to a minimum value of a magnitude of adiscontinuous component that belongs to the surrounding of the blockboundary. The adjusting operation is then applied to a spatial domain.In addition, a DC offset mode is established, and in the DC offset modethe blocking artifact is also reduced, for example, in a smooth regionwhere there is little motion.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a diagram that illustrates horizontal and vertical blockboundaries;

FIG. 2 is a diagram that illustrates a 4-point DCT kernel; and,

FIG. 3 is a flow chart that illustrates a preferred embodiment of amethod that reduces a blocking artifact when coding a moving pictureaccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made to preferred embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. FIG. 1 illustrates typical horizontal and vertical blockboundaries.

As shown in FIG. 1, in the dimensional image formed with respective fourpoints of S₀, S₁, and S₂ located around the block boundary, S₁ and S₂are individually processed with a block-unit compression method. Thus,S₁ and S₂ are not influenced by the blocking artifact. However, S₀ islocated across a block boundary. Thus, S₀ is directly influenced by theblocking artifact. The blocking artifact appears at the boundary betweenfixed block patterns in the form of a line of discontinuity.

Preferred embodiments of the present invention use, for example, afrequency property to preserve complex regions at block boundaries. Thefrequency property around the boundary is preferably obtained by using a4-point DCT kernel, which can be easily calculated. However, the presentinvention is not intended to be limited to this. In this case, thecomplex region at a block boundary can be effectively processed byextending the smoothness of a picture from a frequency domain to aspatial domain.

As shown in FIG. 1, S₀ is located across the block boundary. Thus, S₀ isdirectly influenced by the blocking artifact. To reduce the blockingartifact from S₀, a first preferred embodiment of the present inventionuses frequency information in S₁ and S₂. The blocking artifact can beremoved from S₀ by replacing the frequency component in S₀, which isinfluenced by the blocking artifact, with the frequency components of S₁and S₂. In other words, S₀ contains a discontinuity. However, S₁ and S₂,which are completely included inside respective blocks, are not relatedto the discontinuity. Since S₁ and S₂ are not involved with thediscontinuity at a block boundary, S₁ and S₂ can accurately representfeatures of the respective neighboring blocks.

When images change smoothly, image features of S₀, S₁ and S₂ aresimilar. This means that frequency domains of S₀, S₁ and S₂ have similarfeatures. The preferred embodiments use a DCT, or the like as afrequency analysis tool. DCT is widely used in an image compressiontechnique

FIG. 2 is a diagram illustrating a 4-point DCT basis. As shown in FIG.2, the 4-point DCT kernel basis has symmetric and anti-symmetricproperties around the center of 4 points. In FIG. 2, a₀,0, a₁,0, a₂,0,and a₃,0 are defined as the 4-point DCT coefficients of S₀. Althoughboth a₂,0, and a₃,0 are high frequency components, a₂,0 is symmetric,and a₃,0 is anti-symmetric around the center.

The center of S₀ is located at a block boundary as shown in FIG. 1.Thus, a factor directly affecting the block discontinuity is not thesymmetric component but the anti-symmetric component. The magnitude ofa₃,0 in a frequency domain is thus adjusted based on the anti-symmetriccomponent being a major factor affecting the discontinuity. Accordingly,the proper adjustment of a₃,0 is directly related to the reduction ofblock discontinuity in the spatial domain. Reduction of the blockdiscontinuity will now be described.

In a first preferred embodiment, the magnitude of a₃,0 is replaced withthe minimum value of the magnitudes of a₃,1 and a₃,2, which arecontained in a single block in an area surrounding a block boundary. Bydoing this, a large blocking artifact that appears when one side of theblock boundary to be processed is smooth can be reduced. For a compleximage where both S1 and S2 are the objects of motion (i.e., all thevalues of the magnitudes of a₃,0, a₃,1 and a₃,2 are large), there islittle influence on the block boundary.

A method for reducing a blocking artifact in a default mode is asfollows:

    v.sub.3 '=v.sub.3 -d;

    v.sub.4 '=v.sub.4 +d; and

    d=CLIP(c.sub.2 (a.sub.3,0 '-a.sub.3,0)//c.sub.3,0, (v.sub.3 -v.sub.4)/2)*δ(|a.sub.3,0 |<QP).

In the method, a₃,0 '=SIGN(a₃,0)*MIN(|a₃,0 |,|a₃,1 |,|a₃,2 |), and q isthe component of DCT kernel. The condition |a₃,0 |<QP is used to countthe influence of the quantization parameter on the blocking artifact.The |a₃,0 <QP condition also prevents over-smoothing when the blockingartifact is not very serious. The clipping operation on the compensatedvalue prevents the direction of the gradient at the boundary from beinglarge or changed in an opposite direction. The boundary pixel values, v₃and v₄, are replaced with v₃ ' and v₄ '. QP is the quantizationparameter of the macroblock where v₄ belongs. Values, c₁, c₂, and c₃ arekernel constants used in the 4-point DCT. To simplify an equationaccording to a first preferred embodiment of the present invention, thevalues of c₁ and c₂ are approximated to an integer, and the value of c₃is approximated to a multiple of 2. The values of a₁, a₂, and a₃ areevaluated from the simple inner product of the DCT kernel and pixels,S₀, S₁, and S₃.

    a.sub.3,0 =([c.sub.1 -c.sub.2 c.sub.2 -c.sub.1 ]*[v.sub.2 v.sub.3 v.sub.4 v.sub.5 ].sup.T)/c.sub.3

    a.sub.3,0 =([c.sub.1 -c.sub.2 c.sub.2 -c.sub.1 ]*[v.sub.0 v.sub.1 v.sub.2 v.sub.3 ].sup.T)/c.sub.3

    a.sub.3,0 =([c.sub.1 -c.sub.2 c.sub.2 -c.sub.1 ]*[v.sub.4 v.sub.5 v.sub.6 v.sub.7 ].sup.T)/c.sub.3

Such processes are performed in both horizontal and vertical blockboundaries. In this manner, the blocking artifacts in the whole framecan be reduced.

The first embodiment reduces a blocking artifact in the default mode.However, in the default mode, only the boundary pixel values, v₃ and a₄,are compensated. Thus, the default mode is not sufficient to reduce theblocking artifact in a very smooth region, such as a setting in apicture.

To reduce the blocking artifact in the smooth region, a second preferredembodiment of a method for reducing blocking artifacts in a movingpicture according to the present invention includes a DC offset mode.The method in the DC offset mode is as follows:

    v.sub.3 '=v.sub.3 -d;

    v.sub.4 '=v.sub.4 +d;

    v.sub.2 '=v.sub.2 -d.sub.2 ;

    v.sub.5 '=v.sub.5 +d.sub.2 ;

    v.sub.1 '=v.sub.1 -d.sub.3 ; and

    v.sub.6 '=v.sub.6 +d.sub.3.

In the second preferred embodiment,

    d.sub.1 =(3(v.sub.3 -v.sub.4)//8)*δ(|a.sub.3,0 |<QP),

    d.sub.2 =(3(v.sub.3 -v.sub.4)//16)*δ(|a.sub.3,0 |<QP), and

    d.sub.3 =(3(v.sub.3 -v.sub.4)//32)*δ(|a.sub.3,0 |<QP).

The blocking artifact in the region where there is little motion, orwhich is a very small setting, is reduced through the above-describedmethod or the like in the DC offset mode. An appropriate mode betweenthe DC offset mode and default mode can be determined using on thefollowing conditional expression:

    If (v.sub.0 ==v.sub.1 &&v.sub.1 ==v.sub.2 &&v.sub.2 ==v.sub.3 &&v.sub.4 ==v.sub.5 &&v.sub.5 ==v.sub.6 &&v.sub.6 ==v.sub.7)

DC offset mode is applied; Else Default mode is applied.

When the DC offset mode or the default mode is selected according to theabove conditional expression, the blocking artifacts are reduced in eachmode. After determining the proper mode between the DC offset mode andthe default mode, the block discontinuity at the boundary is compensatedto form a consecutive line, which reduces the blocking artifact. In thesecond preferred embodiment, the DC offset mode and the default mode areset using S₀, S₁ and S₂. However, the present invention is not intendedto be limited to this. Alternative sets of points or the like can beused.

An examplary method for reducing a blocking artifact when coding amoving picture, according to the second preferred embodiment of thepresent invention, is described with reference to the flow chart shownin FIG. 3.

After beginning in FIG. 3, control continues to step 101. In step 101, aplurality of pixels, S0, S1, and S2 are defined centering around a blockboundary. From step 101, control continues to step 102. In step 102, ifa mode is selected, a default mode is set, and control continues to step103.

In step 103, frequency information of the surroundings of the blockboundary for each pixel is obtained using, for example, the 4-point DCTkernel. From step 103, control continues to step 104. In step 104, themagnitude of discontinuous component belonging to the block boundary isreplaced with the minimum magnitude of the discontinuous componentsbelonging to the surroundings of the block boundary in the frequencydomain. From step 104, control continues to step 105, where theadjusting operation is applied to the spatial domain. The default modeis effective in reducing the blocking artifact in a complex region of apicture. However, the default mode is less successful in a smooth regionsuch as a setting in a picture.

Therefore, in a smooth region it is necessary to reduce the blockingartifact in another mode, the DC offset mode. In step 106, the DC offsetmode is established. From step 106, control continues to step 107. Instep 107, the blocking artifact in the region where there is littlemotion, such as a setting, is reduced. From step 107, the process ends.Thus, the overall blocking artifacts can be reduced according to thepreferred embodiments.

As described above, the blocking artifact reduction methods according tothe preferred embodiments of the present invention have variousadvantages and effects. The blocking artifact is more easily andeffectively reduced using features of the frequency domain. Thepreferred embodiments provide a visually finer quality of a picture byreducing the blocking artifacts in both the complex and smooth regions.Further, calculations are simple. Accordingly, the amount of bits doesnot increase.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teaching can bereadily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims. Many alternatives, modifications, and variationswill be apparent to those skilled in the art.

What is claimed is:
 1. A method coding a moving picture using aplurality of blocks, comprising the steps of:selecting a plurality ofpixels positioned around a block boundary; obtaining frequencyinformation for each of the plurality of pixels; adjusting adiscontinuous component in a frequency domain of a first pixel of theplurality of pixels located at the block boundary based on acorresponding component in the frequency domain of a second pixel of theplurality of pixels near the block boundary; and applying the adjustingoperation to a spatial domain of the first pixel to reduce a blockingartifact.
 2. The method according to claim 1, wherein the selectingthrough applying steps are performed in a first mode.
 3. The methodaccording to claim 1, wherein a magnitude of the discontinuous componentin the first pixel is adjusted to a magnitude of the correspondingcomponent in the second pixel, wherein the magnitude of thecorresponding component in the second pixel is based on a smallest valueof corresponding component magnitudes in remaining pixels of theplurality of pixels.
 4. The method according to claim 3, wherein theadjusting step satisfies at least one of the following conditions:

    v.sub.3 'v.sub.3 -d; and

    v.sub.4 '=v.sub.4 +d;

where

    d=CLIP(c.sub.2 (a.sub.3,0 '-a.sub.3,0)//c.sub.3 0,(v.sub.3 -v.sub.4)/2)*δ(|a.sub.3,0 |<QP),

    a.sub.3,0 '=SIGN(a.sub.3,0)*MIN(|a.sub.3,0 |,|a.sub.3,1 |,|a.sub.3,2 |),

wherein v₃ '-v₄ ' are adjusted boundary pixel values, a₃,0 -a₃,2 are thediscontinuous component of the discrete cosine transform coefficients ofthe first and second pixels, c₂ and c₃ are DCT kernel coefficients andQP is a quantization parameter of a macroblock containing v₄.
 5. Themethod according to claim 3, wherein the remaining pixels of theplurality of pixels are positioned within a block adjacent the blockboundary.
 6. The method according to claim 1, furthercomprising:determining a smoothness level of the plurality of pixels;and selecting one of a first and a second mode based on the smoothnesslevel, wherein the blocking artifact is reduced based on the selectedmode.
 7. The method according to claim 6, wherein the second mode isselected when the following condition is satisfied: (v₀ ==v₁ &&v₁ ==v₂&&v₂ ==v₃ &&v₄ ==v₅ &&v₅ ==v₆ &&v₆ ==v₇), wherein v₀ -v₇ are boundarypixel values.
 8. The method according to claim 6, wherein in the secondmode is selected for a region of the motion picture where there islittle motion.
 9. The method according to claim 8, wherein the adjustingstep prevents over-smoothing when the blocking artifact is not veryserious and counts an effect of a quantization parameter.
 10. The methodaccording to claim 6, wherein the adjusting step in the second modesatisfies at least one of the following conditions:

    v.sub.3 '=v.sub.3 -d;

    v.sub.4 '=v.sub.4 +d;

    v.sub.2 '=v.sub.2 -d.sub.2 ;

    v.sub.5 '=v.sub.5 +d.sub.2 ;

    v.sub.1 '=v.sub.1 -d.sub.3 ; and

    v.sub.6 '=v.sub.6 +d.sub.3,

where

    d.sub.1 =(3(v.sub.3 -v.sub.4)//8)*δ(|a.sub.3,0 |<QP),

    d.sub.2 =(3(v.sub.3 -v.sub.4)//16)*δ(|a.sub.3,0 |<QP), and

    d.sub.3 =(3(v.sub.3 -v.sub.4)//32)*δ(|a.sub.3,0 |<QP),

wherein v₀ -v₇ are initial boundary pixel values, v₁ '-v₆ ' are adjustedboundary pixel values, a₃,0 is the discontinuous component of thediscrete cosine transform coefficients of the first pixel and QP is aquantization parameter of a macroblock containing v₄.
 11. The methodaccording to claim 1, wherein the obtaining frequency information stepuses a 4-point discrete cosine transform (DCT) kernel.
 12. The methodaccording to claim 11, wherein the plurality of pixels includes S₀, S₁,and S₂ pixels centered around the block boundary, and whereincorresponding DCT coefficients are determined by an inner product of theDCT kernel and the pixels, S₀, S₁, and S₂.
 13. The method according toclaim 1, wherein the plurality of pixels are centered around the blockboundary.
 14. The method according to claim 1, wherein a first mode is adefault mode and a second mode is a DC offset mode.
 15. A method ofreducing a blocking artifact for use in coding a moving picture,comprising the steps of:defining at least first, second and third pixelsaround a block boundary; setting a default mode, if the default mode isselected; obtaining frequency information for each of the pixels;adjusting a magnitude of a discontinuous component in a frequency domainbelonging to the block boundary to the minimum value of a magnitude of adiscontinuous component in a frequency domain around the block boundary,and applying a result of the frequency domain adjusting operation to aspatial domain; setting a DC offset mode, if a DC offset mode isselected; and reducing the blocking artifact in at least one of thedefault mode and the DC offset mode.
 16. The method according to claim15, wherein the magnitude of the discontinuous component in the firstpixel is adjusted to the minimum value of the magnitude of correspondingcomponents in one of the second and third pixels, wherein the firstpixel is located at the block boundary and the second and third pixelsare located around the block boundary.
 17. The method according to claim16, wherein the adjusting steps in the default mode are:

    v.sub.3 '=v.sub.3 -d; and

    v.sub.4 '=v.sub.4 +d;

where

    d=CLIP(c.sub.2 (a.sub.3,0 '-a.sub.3,0)//c.sub.3,0,(v.sub.3 -v.sub.4)/2)*δ(|a.sub.3,0 |<QP),

    a.sub.3,0 '=SIGN(a.sub.3,0)*MIN(|a.sub.3,0 |,|a.sub.3,1 |,|a.sub.3,2 |),

wherein v₃ -v₄ are initial boundary pixel values, v₃ '-v₄ ' are adjustedboundary pixel values, a₃,0 -a₃,2 are the discontinuous component of thediscrete cosine transform coefficients of the first and second pixels,c₂ and c₃ are DCT kernel coefficients and QP is a quantization parameterof a macroblock containing v₄.
 18. The method according to claim 15,wherein the default mode and the DC offset mode are selected based on asmoothness level of at least the block boundary.
 19. The methodaccording to claim 18, wherein the adjusting steps in the DC offset modeare:

    v.sub.3 '=v.sub.3 -d;

    v.sub.4 '=v.sub.4 +d;

    v.sub.2 '=v.sub.2 -d.sub.2 ;

    v.sub.5 '=v.sub.5 +d.sub.2 ;

    v.sub.1 '=v.sub.1 -d.sub.3 ; and

    v.sub.6 '=v.sub.6 +d.sub.3,

where

    d.sub.1 =(3(v.sub.3 -v.sub.4)//8)*δ(|a.sub.3,0 |<QP),

    d.sub.2 =(3(v.sub.3 -v.sub.4)//16)*δ(|a.sub.3,0 |<QP), and

    d.sub.3 =(3(v.sub.3 -v.sub.4)//32)*δ(|a.sub.3,0 |<QP),

wherein v₀ -v₇ are initial boundary pixel values, v₁ '-v₆ ' are adjustedboundary pixel values, a₃,0 is the discontinuous component of thediscrete cosine transform coefficients of the first pixel and QP is aquantization parameter of a macroblock containing v₄.
 20. The methodaccording to claim 15, wherein discrete cosine transform coefficientsare determined by an inner product of a discrete cosine transform kerneland the pixels.